The 3D printing technology is opening new capabilities and horizons to various engineering disciplines. In the area of antenna theory, such technology facilitates the fabrication process and enables a high level of accuracy in the design especially for non-planar and volumetric structures.
A quadrifilar helix antenna is one of the most common structures that fall under the category of circularly polarized radiating structures with an end-fire radiation pattern. The sense of circular polarization, whether left or right handed, is dependent on the winding direction of the four helical arms that constitute the antenna. It also depends on the 90-degree progressive phase shift between the four arms. On the other hand, the antenna's ground plane plays a role in shaping and directing the radiation pattern.
Different miniaturization techniques have been used for this type of antennas. These techniques include the reduction of the antenna height by winding the helical arms using sinusoidal profiles, periodic triangular, fractal profiles or nonlinear functions. The implementation of stepped-width helical arms, the incorporation of dielectric rods, the variation of the pitch and turn angles of the helices, the insertion of gaps as well as the folding of the helical arms have been used for miniaturization purpose.
In other efforts, a folded printed quadrifilar helix antenna wherein a miniaturization of about 43% of the antenna's axial length is obtained by meandering and turning the helix arms into the form of square spirals. A length reduction of 15% is achieved without significantly degrading the antenna's bandwidth. The length reduction is attained by modifying the geometry at the center of the helices through the insertion of a gap and folding a short length of the conductor around the cylindrical surface.
In yet other efforts, the turn angles of the helices are employed for the axial length reduction of a half wavelength quadrifilar helix antenna. The use of a combination of sinusoidal profiles for the helical wires has been used.
Various miniaturization techniques have also been implemented on different antenna structures other than the quadrifilar helix. A volumetric miniaturization may be achieved through the adoption of a combination of z-directed meandering of a spiral antenna in addition to a tapered substrate profile. The use of interdigitated slits on a CPW-fed ring slot antenna has also been used. The addition of multiple slits enables the antenna to approximately reach the electrically small limit. The design of complementary split-ring resonators placed horizontally between the microstrip antenna and ground plane has also been shown to be a possible miniaturization technique.